Acrylic flexible light pipe of improved thermal stability

Abstract

Acrylic light pipe as described in Bigley et al., U.S. Pat. Nos. 5,406,641 and 5,485,541, has adequate thermal stability for many purposes. It has been found that improved thermal stability, as reflected in color formation, can be imparted by adjusting the polymerization conditions to produce the uncured core polymer of the core/clad construction with a much reduced terminal vinyl content, preferably below 0.5 vinyl groups/1000 monomer units.

We claim: 
     
       1. A crosslinkable core mixture for a subsequently-cured composite which mixture contains a thermoplastic core polymer, the thermoplastic core polymer having a weight average molecular weight from about 2,000 to about 250,000 daltons and a vinyl end-group content of below 0.5 per 1000 monomer units, the core mixture comprising (a) a thermoplastic core polymer comprising i) from 80 to 99.9 weight percent of polymerized units of a C 1  -C 18  alkyl acrylate or mixtures thereof with up to 50 weight percent of the C 1  -C 18  alkylacrylate of polymerized units of a C 1  -C 18  alkyl methacrylate;   ii) from 0.1 to 18.2 weight percent of polymerized units of a functionally reactive monomer, and   iii) from 0 to about 10 weight percent of polymerized units of a refractive index increasing monomer selected from styrene, benzyl acrylate, benzyl methacrylate, phenylethyl acrylate or phenylethyl methacrylate;   iv) 0.002 to 0.3 weight percent residual molecules of or of decomposition products of an initiator of polymerization, including end groups on the thermoplastic core polymer, the initiator having a half-life at 60° C. of 20 to 400 minutes;   v) from 0.2 to 2.0 weight percent of residual molecules of or of decomposition products of a chain transfer agent, including end groups on the thermoplastic core polymer;     (b) from 0.1 to 10 weight percent, based on the crosslinkable core mixture weight, of a reactive additive.   
     
     
       2. The crosslinkable core mixture of claim 1, further containing a cladding polymer which surrounds the core mixture. 
     
     
       3. The crosslinkable core mixture of claim 2 wherein the cladding polymer is a fluoropolymer, and wherein the crosslinkable core mixture within the extruded fluoropolymer cladding and the extruded fluoropolymer cladding are in substantially complete contact. 
     
     
       4. The crosslinkable core mixture of claim 3 wherein the percentage of polymerized units of a C 1  C 18  alkyl acrylate is 80 to 99.5 weight percent ethyl acrylate, wherein the chain transfer agent is an aliphatic mercaptan of from one to twenty carbon atoms, and wherein the initiator of polymerization is an azo compound. 
     
     
       5. The crosslinkable core mixture of claim 3 wherein the functionally reactive monomer is present at a level of from about 0.5 to about 12 weight percent and is selected from 2-methacryloxyethyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, or mixtures of these, and wherein the reactive additive is water and a silane condensation reaction catalyst. 
     
     
       6. The crosslinkable core mixture of claim 5 wherein the silane condensation reaction catalyst is a dialkyltin dicarboxylate. 
     
     
       7. The crosslinkable core mixture of claim 6 wherein the uncrosslinked copolymer is formed from polymerized units of 94 to 98 weight percent ethyl acrylate and from about 2 to about 6 weight percent of 3-methacryloxypropyltrimethoxysilane, and the silane condensation reaction catalyst is dibutyltin diacetate. 
     
     
       8. A flexible light pipe product containing the crosslinked core mixture of claim 2, wherein the product has good light transmittance wherein the differential transmission loss between light wavelengths of 400 nm to 600 nm is equal to or less than 1.0 decibel per meter as measured by a "cut-back" interference filter method; excellent thermal stability, wherein a change in the differential transmission loss between light wavelengths of 400 nm to 600 nm is equal to or less than 1.0 decibel per meter after >100 hours of exposure to a temperature of 120° C., as measured by a non-destructive interference filter method; good flexibility, wherein the product, at 20° C., survives without core fracture a 180° bend at a bend radius which is less than or equal to five times the diameter of the cured core; and good hardness properties, wherein the Shore "A" hardness is less than 90 after 50 days of exposure at 120° C. 
     
     
       9. A process for preparing a crosslinkable core mixture for a subsequently-cured composite comprising a coextruded cladding polymer and a coextruded crosslinkable core mixture, which mixture contains a thermoplastic core polymer having a weight average molecular weight from about 2,000 to about 250,000 daltons and a vinyl end-group content of below 0.5 per 1000 monomer units, the process comprising a.) preparing an admixture of i) from about 80 to about 99.9 weight percent of a bulk monomer mixture selected from a C 1  -C 18  alkyl acrylate or mixtures thereof with up to 50 weight percent of the bulk monomer mixture of a C 1  -C 18  alkyl methacrylate;   ii) from about 0.1 to about 18.2 weight percent of a functionally reactive monomer, and   iii) from 0 to about 10 weight of a refractive index increasing monomer selected from styrene, benzyl acrylate, benzyl methacrylate, phenylethyl acrylate or phenylethyl methacrylate;     b) adding 0.002 to 0.3 weight percent, based on the uncrosslinked copolymer weight, of an initiator of polymerization which has a half-life at 60° C. of 20 to 400 minutes;   c) prior to, simultaneously, or after the addition of the initiator, adding 0.2 to 2.0 weight percent, based on the uncrosslinked copolymer weight, of a chain transfer agent;   d) charging the monomer admixture, initiator, and chain transfer agent reaction mixture to a constant-flow stirred reactor heated to 70-120° C., to form a polymerized, non-crosslinked, crosslinkable core mixture to a devolatilizing apparatus to remove unreacted monomers;   e) prior to, during, or after the devolatilization and/or co-extrusion, adding from 0.1 to 10 weight percent, based on the crosslinkable core mixture weight, of a reactive additive;   f) coextruding the crosslinkable core mixture and the cladding polymer to form a curable composite.   
     
     
       10. The process of claim 9 wherein the coextruded cladding polymer and a coextruded crosslinkable core mixture are continuously, concurrently and coaxially extruded, wherein the cladding polymer is a molten fluoropolymer, wherein the extruded crosslinkable core mixture within the extruded fluoropolymer cladding and the extruded fluoropolymer cladding are in substantially complete contact after filling the extruded tubular cladding with the extruded crosslinkable core mixture, wherein the curing is conducted subsequently and separately from the extrusion and cladding operation, and wherein a portion of the reactive additive is added to the core mixture after the co-extrusion. 
     
     
       11. A flexible light pipe product by the process of claim 9 or 10 wherein the product has good light transmittance wherein the differential transmission loss between light wavelengths of 400 nm to 600 nm is equal to or less than 1.0 decibel per meter as measured by a "cut-back" interference filter method; excellent thermal stability, wherein a change in the differential transmission loss between light wavelengths of 400 nm to 600 nm is equal to or less than 1.0 decibel per meter after >100 hours of exposure to a temperature of 120° C., as measured by a non-destructive interference filter method; good flexibility, wherein the product, at 20° C., survives without core fracture a 180° bend at a bend radius which is less than or equal to five times the diameter of the cured core; and good hardness properties, wherein the Shore "A" hardness is less than 90 after 50 days of exposure at 120° C.